My car was recently broken in to and a piece of a spark plug was found by the cop. He told me this is what thieves use to break car windows. It makes almost no sound at all like a brick or a hammer would. It doesn't even need to be thrown hard.
Why does tempered glass shatter so easily when it is hit (even when lightly tossed) with a piece of porcelain from a spark plug or an electrical insulator? What inherent properties collide or reaction causes this?

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"Life is like a shit sandwich. The more bread you have, the less shit you have to eat.

Padeye

02-03-2000, 06:23 PM

Tempered safety glass is quite difficult to break with a blunt heavy blow but easy to shatter with something sharp and hard. The tempering puts the surface under stress which makes resistant to breaking but causes it to break into small relatively round bits when it does instead of long splintery shards.

I tried to break a window out of a car I junked with ball pien hammers, sleges, rockc, etc. and couldn't do a thing to it. I tried something I saw on a TV program, an automatic center punch, and pow! Instant glass BBs, sorta. The center punch idea was presented as an escape tool if your car went in a canal or something. Makes sense that the ceramic of a spark plug could be used the same way.

I can understand about the centerpunch being able to break it easily since the punch is spring-loaded and usually has a carbide or diamond tip. But what is it about porcelain that doesn't need much force upon impact to break the glass? How come when you throw a stone (small of course and not with much force) that has some sharp edges, not break?

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"Life is like a shit sandwich. The more bread you have, the less shit you have to eat.

Cartooniverse

02-03-2000, 07:41 PM

We use a spring-loaded centerpunch to get into cars at accident scenes. There's not "spray" of glass inwards, it just kinda collapses. I cannot IMAGINE why porcelain would do that. Perhaps, you are making an assumption in error? At the other end of the spark plug...is...a very sharpt metal point, n'est pas?
As for the OP Name, I have cats, and they have some questions about Distempered glass :)

Cartooniverse

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If you want to kiss the sky, you'd better learn how to kneel.

YTDread

02-03-2000, 09:25 PM

Well, since my first post this afternnon I called my buddy who owns a towing business and asked him if he had a "donor" car. I went over (porcelain in hand) and proved the theory for myself. IT WORKS! Just toss a broken piece of spark plug insulator at a temepered window and you'll find out. No explosion, just the window "freezing" and a second later, millions of bits of glass falling down. WOW! Now I ask again what is it about this stuff that causes this reaction??? Thanks for the replies.

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"Life is like a shit sandwich. The more bread you have, the less shit you have to eat.

Zor

02-03-2000, 11:10 PM

Glass is a material with relatively high tensile strength. When pressure is applied to it, the force is spread uniformly across its surface and can be absorbed with ease. This property, however, also makes glass extremely susceptible to structural damage. A small scratch alone will cause large amounts of stress to build up around it as an incoming force is being conducted across the surface. Once the glass can no longer withstand the stress at that particular point, it will start to crack right there, enlarging the damage, which leads to even more cracking. As this goes into positive feedback, the end result is the dramatic shattering of glass we are familiar with.

So, in order to break a piece of glass efficiently, you'll not only need brute force, but also some mechanism of causing a bit of structure damage. On the Mohs' scale of hardness between 1 and 10, ordinary glass usually stands between 4 and 5. Diamonds, carbide, and ceramics (which include porcelain) all rate between 9 and 10, and are thus easily capable of damaging glass.

By the way, if you ever need to break one of those large reinforced glass windows, and you only have blunt objects nearby, your best bet is to bash away at its corners instead of its center. Quite a few of people have died in fires because they couldn't figure out a way to break down the windows and escape...

YTDread

02-04-2000, 06:24 PM

Thank you. That's what I needed to know.

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"Life is like a shit sandwich. The more bread you have, the less shit you have to eat.

Arnold Winkelried

02-04-2000, 08:04 PM

Originally posted by YTDread:
Just toss a broken piece of spark plug insulator at a temepered window and you'll find out. No explosion, just the window "freezing" and a second later, millions of bits of glass falling down.

OK, I want to know more. How big a piece? How hard did you throw it? I still find it incredible that hitting a window as hard as you can with a hammer won't break it (I know, I tried on my car once when I locked the keys inside), but tossing a small piece of ceramic from a spark plug will cause it to break instantly.

YTDread

02-04-2000, 08:14 PM

Thank you. That's what I needed to know.

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"Life is like a shit sandwich. The more bread you have, the less shit you have to eat.

YTDread

02-04-2000, 08:20 PM

The piece was pea size (I used a piece similar to the one that was found) and was thrown with the force a little more than you would flick a cigarette at the ground. Try it. Weird, I know. But I promise it works.